The present invention relates to an ultrasonic diagnostic apparatus, and particularly to a technique of generating shear waves in a living body using acoustic radiation force to evaluate the elasticity using the propagation velocity.
Medical image display apparatuses typified by ultrasonic waves, MRI (Magnetic Resonance Imaging), and X-ray CT (Computed Tomography) have been widely used as apparatuses that present information in a living body that cannot be visually confirmed in the form of values or images. Among those, an image display apparatus using ultrasonic waves is provided with a high degree of temporal resolution as compared to the other apparatuses, and has performance capable of imaging a pulsating heart without blurring.
Ultrasonic waves propagating in a living body are mainly classified into longitudinal waves and transverse waves, and information of the longitudinal waves (a sound velocity of about 1540 m/s) is mainly used in many techniques used in products, namely, techniques of visualizing tissue configurations and measuring a blood velocity.
Recently, a technique of evaluating the elastic modulus of a tissue using transverse waves (hereinafter, referred to as shear waves) attracts attention, and is being used for mammary tumors and chronic liver disease in clinical practice. In the technique, shear waves are allowed to be generated inside a tissue as a measurement target, and the elasticity is evaluated on the basis of the propagation velocity. The techniques of generating the shear waves are roughly classified into a mechanical method and a radiation pressure method. The mechanical method is a method in which a vibration of about 1 kHz is applied to a body surface using a vibrator or the like to generate the shear waves, and a driving apparatus serving as a vibrating source is necessary. On the other hand, in the radiation pressure method, acoustic radiation pressure is applied to the inside of a living body using focused ultrasonic waves that allow ultrasonic waves to be locally concentrated in a tissue, and the shear waves are allowed to be generated using following tissue displacement. Each method is a technique in which the tissue displacement caused by propagation of the generated shear waves is measured using ultrasonic waves to evaluate information related to hardness of the tissue.
As prior art documents related to these techniques, for example, U.S. Pat. No. 8,118,744B2 and US2010/0222678A1 relate to a method of an elastic evaluation using acoustic radiation pressure.